Commercial Cargo Delta V Question for ISS

[pathfinder_01]

Dragon masses 4200kg (dry) and can support 6000kg of up-mass to LEO with 1290 kg of flight propellant. That sounds like 11,490kg total (max), but that means it won't make it to a 400km height for ISS, which means removing upmass. Maximum upmass would then be (9358-4200-1290)=3868kg upmass capable only.

Hmmm. Sound correct?
 

Might not be carring 1290kg of propellant. Dragon only needs enough propellant to rendevous with the ISS. Dragon is built to stay 2 years in space without the ISS so I would expect that it would need extra propellant for station keeping.

[Danderman]

Something that maybe one of our SpaceX 'extremists'    can answer (since I'm useless at these things):

From the Falcon 9 user's guide, it can put 9358 kg in a 400km x 51.6 deg inclination circular LEO from Cape Canaveral.

Hmmm. Sound correct?

The COTS Dragon missions don't require a payload fairing or LAS, so maybe the F9 user guide number is not valid for those missions, which might give the 10K kg payload mass for COTS missions that has been bandied about.

I was thinking that 1290 kg prop is excessive for ISS missions, but its virtually exactly the same prop load as Soyuz, adjusted for the greater Dragon mass. Soyuz has margin to achieve the 425 km altitude required for the ISS orbit, so Dragon should be OK.

[robertross]

Dragon masses 4200kg (dry) and can support 6000kg of up-mass to LEO with 1290 kg of flight propellant. That sounds like 11,490kg total (max), but that means it won't make it to a 400km height for ISS, which means removing upmass. Maximum upmass would then be (9358-4200-1290)=3868kg upmass capable only.

Hmmm. Sound correct?
 

Might not be carring 1290kg of propellant. Dragon only needs enough propellant to rendevous with the ISS. Dragon is built to stay 2 years in space without the ISS so I would expect that it would need extra propellant for station keeping.

Well that's partially correct, but you also need propellant to depart the ISS, not just rendezvous. They may offload some propellant on later missions, once all the systems are refined. They may also do some changes to increase upmass like they did with ATV-II vs. ATV-I.

[QuantumG]

There's no numbers for Dragon.. dry mass or otherwise.  The 4200kg thing is from some ancient literature before there was even one built.  Maybe after the next COTS demo they'll actually publish some real numbers.

[Lurker Steve]

8000lbs versus 19000lbs is a LOT of less logistics, too!

Half of the market for commercial resupply, evaporated ..

That means fewer Progress vehicles are needed to refuel the thrusters on the Russian side of the station.

It doesn't mean that the station will require less food, water, clean clothes, spare parts, etc that get transported up in the pressurized modules.

[Robotbeat]

8000lbs versus 19000lbs is a LOT of less logistics, too!

Half of the market for commercial resupply, evaporated ..

That means fewer Progress vehicles are needed to refuel the thrusters on the Russian side of the station.

It doesn't mean that the station will require less food, water, clean clothes, spare parts, etc that get transported up in the pressurized modules.

Quite right. Commercial resupply doesn't (or, rather, isn't supposed to) supply reboost propellant, that's only Shuttle (reboost), ATV, and Progress. This means less need for Shuttle reboost (and improved microgravity).

[arkaska]

I think he meant that the vehicles currently carrying propellant can carry other stuff instead freeing up the need for as much commercial cargo. But as I sad before, this has been the plan for a long time so if they don't raise the orbit they will need MORE vehicles going up to ISS.

[simonbp]

What is this vehicle called "Delta V" and what does it have to do with the ISS? 

I think it's Delta IV + I, and I don't know.

Yeah, a Delta IV CBC with two Extended-tank Thors slaped on either side. Now that's a rocket...

[Space Pete]

I think he meant that the vehicles currently carrying propellant can carry other stuff instead freeing up the need for as much commercial cargo. But as I sad before, this has been the plan for a long time so if they don't raise the orbit they will need MORE vehicles going up to ISS.

The commercial vehicles will need to carry MORE propellant in order to rendezvous with the ISS in its higher orbit. That means they will be able to carry LESS dry cargo than they would if ISS was in a lower orbit. If they were to carry the same amount of dry cargo, then they would need even MORE propellant.

[Robotbeat]

I think he meant that the vehicles currently carrying propellant can carry other stuff instead freeing up the need for as much commercial cargo. But as I sad before, this has been the plan for a long time so if they don't raise the orbit they will need MORE vehicles going up to ISS.

The commercial vehicles will need to carry MORE propellant in order to rendezvous with the ISS in its higher orbit. That means that they will have to carry LESS dry cargo than they would if ISS was in a lower orbit. If they were to carry the same amount of dry cargo, then they would need even MORE propellant.

What difference in payload? 1-5%? Dragon should have no problem, since Dragon only has to do about 1667kg per flight, while it has a capacity far more than double that (if you include both pressurized and unpressurized). If cargo is volume-limited, then this should make no difference.

[arkaska]

The commercial vehicles will need to carry MORE propellant in order to rendezvous with the ISS in its higher orbit. That means they will be able to carry LESS dry cargo than they would if ISS was in a lower orbit. If they were to carry the same amount of dry cargo, then they would need even MORE propellant.

My point was that this reduced need of propellant will not mean they have a have more up-mass then needed since this have been the plan for a long time.